Photometric circuit

ABSTRACT

In an instrument for the detection of ultra-violet radiation, visible light or infrared radiation an electric coupling is known, which comprises a photo-diode of silicon or germanium, a measurement resistor, over which generated photo-electric current generates a voltage, and a voltmeter with low leakage current for measuring the voltage generated. For rendering possible a less expensive, simpler and safer structural design than the designs known and commercially available, it is proposed that the measurement resistor R A  and the calibrated voltmeter V each are connected in parallel over the photo-diode.

CROSS REFERENCE TO RELATED APPLICATION(S)

This United States application stems from PCT International Application No. PCT/SE 86/00325 filed July 4, 1986.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to an electric circuit in an instrument for detecting ultra-violet radiation, visible light or infrared radiation, comprising a photo-diode of silicon or germanium, a precision resistor for measuring, via which generated photo-electric current from the photo-diode generates a voltage, and a voltmeter with low leakage current for measuring the voltage generated.

The present invention has as an object to render possible a structural design, which is less expensive, simpler and safer than the ones known and commercially available.

The invention is characterized in that the precision resistor and calibrated voltmeter each are connected in parallel with the photo-diode.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in the following with reference to the accompanying drawings, in which

FIG. 1 shows schematically a circuit diagram for a known photometric circuit,

FIG. 2 shows the current-voltage characteristic of the photo-diode in the forward direction, and

FIG. 3 shows a circuit diagram according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The cicuit shown in FIG. 1, comprises a photo-diode F and a voltmeter connected in parallel therewith. Before the voltmeter an operational amplifier O and a resistance R_(A) are connected. The resistance can be exchanged with resistances of a different size for changing the measurement ranges. The circuit acts in such a way, that the photo diode F at exposure to the radiation E generates in its backward direction a photo-electric current I_(F). The photo-electric current I_(F) is passed on via the amplification resistor R_(A) of the operational amplifier O. The generated voltage is indicated by the voltmeter V. This voltage is proportional to the radiation E provided that the resistance trimmer T is adjusted so that

    V=O when E=O.

As this active amplification system is temperature-sensitive, the adjustment must be made manually or automatically at considerable cost.

In FIG. 2 the current-voltage characteristic of the photo-diode is shown in the forward direction, i.e. opposed to the direction of the photo-electric current generation. It appears from FIG. 2 that at low voltage V in the forward direction the diode acts as an insulator. The resistance dV/dI is very high, near the origin of the coordinates. This applies especially to silicon diodes.

FIG. 3 shows the circuit according to the present invention. The voltmeter and the measurement resistor R_(A) both are connected in parallel with the diode F. The measurement resistances can be exchanged automatically for changing the measurement ranges. The resistor R_(L) illustrates the resistance in the barrier layer of the photo-diode in the conducting direction, i.e. slope dV/dI of the curve in FIG. 2.

I_(F) is the photo-electric current in the backward direction of the diode generated by the radiation E. I_(L) is the leakage current generated by the voltage V where

    V=R.sub.A (I.sub.F -I.sub.L -I.sub.V).

The leakage current is in the forward direction. I_(V) is the very low leakage current through the modern voltmeter V. The magnitude for for the leakage current is one pico-ampere.

When the generated voltage V across the resistor R_(A) is limited to the amplitude range ΔV in FIG. 2, below which the backward resistance R_(L) can be regarded as much higher than R_(A), therefore the leakage current I_(L) is much less than the photoelectric current I_(F), and when I_(V) can be regarded as much lower than I_(F), it applies that I_(F) -I_(V) -I_(L) is approximately equal to I_(F) and, thus, the generated voltage is approximately

    V=I.sub.F ×R.sub.A.

The voltmeter is adjusted so that maximum deflection is attained at a voltage equal to or lower than ΔV. Calibration with a known radiation E can be carried out directly by adjustment of the voltmeter when R_(A) is chosen so that I_(F) ×R_(A) is lower than ΔV.

For a silicon diode with a surface of some few mm² (e.g., 2.5 mm×2.5 mm) and adequate filtering, I_(F) is approximately equal to 100 pA/lux or I_(F) is approximately equal to 1000 pA/W/m² for ultra-violet or infrared radiation. At a voltage V=50 mV, R_(A) quite simply can be chosen as 10MΩ without any non-linearity in the relation

    V=k×E.

being noticeable, where k is a constant.

With this circuit, this, illumination intensities from 0.1 lux or radiation intensities in ultra-violet or infrared from 0.01 w/m² can be measured. For photo-diodes with a different surface area ΔV is to be chosen so that

    V=k×E

constantly is substantially linear. At the measurement of higher radiation intensities R_(A) is changed to lower resistance values R_(A) (N) so that the voltage always is limited to ΔV or below.

The circuit according to the present invention implies substantial savings in material and labor costs and a superior simplicity of handling owing to the automatic zero-point stability. The voltmeter certainly may contain active amplification, but due to high-technology, the mass production modern voltmeters are zero-point stable at varying temperatures. Modern voltmeters, generally digital ones, also are well able to resolve voltages as small as have been reported here (for example 50 mV in two-thousand scale divisions). 

I claim:
 1. An electronic circuit for an instrument for indicating the intensity of light radiation, comprising:a photo-diode means for generating a photo-electric current in response to light radiation incident thereon; a measurement resistance means connected in parallel with the photo-diode means, said photo-electric current generating a voltage across said measurement resistance means, the amplitude of said voltage corresponding to the intensity of the incident light radiation; and a low leakage current voltmeter means connected in parallel with the photo-diode means and the measurement resistance means for measuring the voltage across the measurement resistance means which voltage across the measurement resistance means is equal to or less than a voltage amplitude at which a varying forward internal resistance of the photo-diode means has a substantially higher value than that of the measurement resistance means, thus making the leakage current of said photo-diode means substantially less than the current through said measurement resistance means; such that the voltage amplitude across the measurement resistance means measured by the voltmeter means corresponds linearly to the photo-electric current generated by the photo-diode means. 